Originally used mostly by cornea specialists to detect irregular astigmatism and changes in curvature after corneal surgery, as well as an aid in contact lens fitting, corneal topography is becoming an increasingly more important test for ophthalmologists.
When laser vision correction became popular, corneal topography was necessary for screening refractive surgery candidates for contact lens-related warpage and corneal ectasias, as well as for analyzing postoperative corneas. Now with refractive cataract surgery, advanced technology intraocular lenses, and higher patient expectations for visual outcomes, corneal topography is essential for determining preexisting abnormalities such as contact lens warpage, keratoconus, pellucid marginal degeneration, previous refractive surgery, anterior basement membrane dystrophy and dry eye. These imaging technologies are also critical when determining the appropriate IOL for the patient and the best method for astigmatism correction.
Traditionally, corneal curvature is measured with a keratometer. This is most commonly performed with a manual instrument that records two anterior corneal curvature values (maximum and minimum) 90 degrees apart. These devices have major limitations because they only measure a small area of the paracentral cornea (two points at the 3-4 mm zone) and they assume that the shape of the cornea is a symmetric spherocylinder with a major and minor axis separated by 90 degrees. Despite these drawbacks, manual keratometry provides accurate readings for most patients, and it has been the standard method for determining astigmatism for IOL calculations and contact lens fitting.
However, corneal topography provides more data, quantifies corneal shape, and measures patterns of irregular and induced astigmatism. Therefore, keratometry readings are frequently performed with automated devices such as corneal topographers, autorefractors, and optical biometers such as the IOLMaster 500 from Carl Zeiss Meditec, and LenStar 900 from Haag-Streit because of the ease, speed, and additional information given by these instruments which most ophthalmologists now have in the office.
Corneal topographers are usually classified according to the imaging technology they utilize, such as placido-based (videokeratoscopy), elevation-based (rastersphotogrammetry, scanning slit), and interferometry-based (laser holography, Moiré fringes). The data is presented as various types of maps: curvature (axial, instantaneous), power (refractive), elevation, difference, or relative maps. Modern topography systems also include qualitative classification systems and quantitative measures, indices, and algorithms to help in data analysis.
The most familiar of these are the simulated keratometry values (Sim K; an estimate of the curvature at the 3 mm zone), but the Sim K readings are less accurate than true keratometry readings and should not be substituted for them. The other very common data analysis measure is the keratoconus screening, which identifies maps that are suspicious for keratoconus and other corneal ectasias. Other software modules include the Holladay Diagnostic Summary, Advanced Refractive Diagnostic, VisionPro (VISX Custom CAP), STARS (healing trend), MasterFit (contact lens), and Paragon CRT Lens Selection programs.
When purchasing a corneal topography device it can be challenging because of the numerous options. For general screening purposes, a basic corneal topography machine is sufficient. This will provide all the basic maps as well as software for keratoconus detection and contact lens fitting. A basic unit also is adequate for doctors who already have some of the components such as OCT, Scheimpflug cameras, wavefront aberrometers, pachymeters, or autorefractors that are now integrated into the new topography machines. If you do not have these other devices and want a system that can perform multiple functions, one of the combined topography systems may be a better choice.
Here are the features of some of the latest corneal topography devices:
TMS-5 Corneal Topographer from Tomey— This topographic modeling system combines a Placido ring topographer and a 3D Scheimpflug camera to give cross sectional imaging of the anterior segment such as anterior topography, posterior topography, anterior chamber analysis, dimensional measurements, and pachymetry. The unit also includes keratoconus screening software.
KR-1W Wavefront Analyzer from Topcon Medical Systems— This device is a 5-in-1 instrument featuring wavefront aberrometry, topography, keratometry, pupillometry, and autorefraction. The system has full auto-alignment, a large color touch screen, simulated visual acuity assessment, and is fully networkable in various configurations.
OPD-Scan III from Marco— This device is a refractive power/corneal analyzer that features autorefraction, keratometry, placido disc topography, wavefront aberrometry, and pupillometry. The system uses auto alignment and auto tracking. The software allows numerous map and wavefront data displays. Other measurements include lenticular residual astigmatism, angle kappa, pre/post toric IOL measurements, and IOL tilt/decentration. It is compatible with EMR and network integration.
MM1 Magellan Mapper Corneal Topographer from Nidek— This topographer features a small diameter ring projector to enhance ring quality and avoid artifacts. The software allows displays of single, multiple, and 3D maps with any scale and map algorithm. The device also includes the Nidek Advanced Vision Information System (NAVIS), an interface that enables easy networking and management of image information.
Visante omni from Carl Zeiss Meditec— This unique device is the first system to combine OCT and placido disc technology to provide evaluation of the cornea and anterior segment with anterior topography, posterior topography, full-width anterior segment imaging, anterior chamber angle measurement, and pachymetry. The V-Trac registration system ensures accurate alignment. Results are summarized on a single page with the Holladay Report.
Atlas 9000 from Carl Zeiss Meditec— This topographer uses the cone-of-focus alignment system to ensure accurate focusing of the Placido disk technology. The SmartCapture technology analyzes and automatically selects the best image during alignment. PathFinder II software evaluates images to identify abnormal topographies, and MasterFit II software is helpful for contact lens fitting of rigid gas permeable lenses, particularly in challenging cases.
Easygraph Topographer from OCULUS— This topographer is a small portable device that can be mounted on a slit-lamp. It also has a built in keratometer. Keratoconus and contact lens fitting software is provided. OxiMap is unique software that provides a color-coded image that displays the oxygen transmissibility of contact lenses.
Corneal Analysis System 3000 from EyeSys— This device features corneal topography, pupillometry, high-resolution 3-camera technology, intuitive user interface, and state-of-the-art image processing software.